Method for producing an optical transmitting and receiving device and a transmitting and receiving device produced according to said method

ABSTRACT

The invention relates to a method for producing an optical transmitting and receiving device ( 1, 1   a ) comprising a light emitting transmission element ( 3, 3   a ) and a receiving element ( 4, 4   a ) which converts this light into an electrical magnitude. The transmission and receiving elements are inserted into a silicon substrate. The optical transmitting and receiving device ( 1 ) is preferably inserted in a monolithic manner into a common substrate, comprising a sequence of superimposed layers for the light emitting transmission element ( 3 ) and the light receiving element ( 4 ). An electrically insulating intermediate layer ( 9, 9   a ) is incorporated between the transmission and receiving element.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing an opticaltransmitting and detecting device including a light-emittingtransmitting element as well as a detecting element to convert thislight into an electrical quantity, the transmitting and detectingelement being incorporated into a silicon substrate.

A known approach exists in which this type of transmitting and detectingsystem is arranged sequentially on a frame to form an optocoupler, butthe approach is complex due to the associated process of bending andprecise assignment. Electrical insulation is created between thetransmitting and detecting systems by providing, for example, anintermediary plastic or lacquer or similarly optically-transparentmaterial.

The publication by S. M. Sze, Physics of Semiconductor Devices, 2^(nd)edition, page 699 discloses an optocoupler which has an LED as theoptical transmitting element and a silicon phototransistor as the lightdetector arranged on both sides of a glass insulator. Here again, acomplex assembly technology is required both for production and foraligning an optical coupling which significantly affects the cost ofproduction.

Optical transmitting elements (LEDs) are constructed on the basis ofgallium arsenide, gallium arsenide phosphide, or gallium phosphide duein part to the high efficiency achieved. However, a disadvantage ofthese compound semiconductors is their poor mechanical properties incomparison to silicon, as well as the problem of integration intosilicon-based systems.

To integrate optical systems based on GaAs, GaAsP, or GaP, the approachhas also been suggested of incorporating a receptacle recess in thesilicon substrate in which the optical system is then inserted and, forexample, joined to the silicon system by bonding. An optical link canthen be created, for example, by lateral emission from the opticalgallium arsenide system to the silicon system. This approach toorequires a complex assembly technology.

The publication Sensors and Actuators A 31 (1992) pp. 229-240 disclosesan optocoupler in which the light source is an avalanche silicon diode.The optocoupler has arranged adjacent to one another on a substrate theavalanche diode as the light source and the light detector in the formof a photodiode. This arrangement requires a comparatively large chipsurface area. In addition, the transmitter and detector are notdielectrically separated. Finally, certain measures are required tofacilitate the lowest-possible-loss transmission of light between thetransmitter and the detector another factor increasing the complexity ofproduction.

Therefore, there is a need for a relatively low cost opticaltransmitting and detecting device, as well as a device that utilizes arelatively small amount of chip area, and a method of producing such adevice.

SUMMARY OF THE INVENTION

Briefly, according to an aspect of the invention, an opticaltransmitting and detecting device is monolithically incorporated into acommon substrate having a stacked sequence of layers for thelight-transmitting element and the light-detecting element, and that anelectrically insulating interlayer is inserted between the transmittingelement and the detecting element.

The space requirement for the chip area is considerably reduced (e.g.,almost to half) due to the sandwiched stacked transmitting and detectingelements, while the spacing between the two elements is maintained assmall as possible, thereby reducing transmission losses, orsignificantly enhancing transmission efficiency. As a result, the powerof the transmitting element may be relatively small without thisnegatively affecting functionality. Specifically, the reduced lightintensity of the silicon-based transmitting element, as compared togallium arsenide systems, is still sufficient for reliable functioningeven given low operating currents.

Another advantage is the electrical separation of the systems by theinsulating interlayer—a required feature when using the device, forexample, as an optocoupler.

In this first proposed approach for producing an optical transmittingand detecting device according to the invention, the transmitting anddetecting elements are incorporated into a common substrate.

According to another independent proposed solution, thelight-transmitting element and light-detecting element may each beincorporated into a silicon substrate, wherein at least one of the twotransmitting and detecting chips thus formed is modified on thelight-emitting or light-detecting side. The modification includes theinsertion of a cavity into at least one of these sides, and the chipsbeing joined with the light-emitting and the light-detecting sidesfacing each other.

A preferred use of this method is when modifications must be madebetween the transmitting and the detecting element such as creatinginterlayers, inserting a cavity, and the like. Since initially twosystems are present on separate substrates, those sides which will laterbe joined and face each other are more easily accessible for suchmodifications—with the result that this production method offerssimplification and advantages in terms of special modifications.

Preferably, the transmitting chip and detecting chip may be joined insandwiched stacked form by bonding. Chip bonding is a common methodavailable in a number of variants.

An optical transmitting and detecting device produced based on themethod according to the invention including a light-emittingtransmitting element as well as a detecting element to convert thislight into an electrical quantity, the transmitting and the detectingelement being incorporated into a silicon substrate.

In a first embodiment, the transmitting and detecting elements arearranged stacked in a common silicon substrate, and a dielectricinterlayer is arranged between the transmitting element and thedetecting element.

In a second embodiment, one silicon chip each is provided for thelight-emitting element and for the light-detecting element, these chipsbeing joined with the light-emitting and the light-detecting sidesfacing each other, and at least one cavity and/or at least oneinsulating layer and/or at least one wavelength-selective layer beingprovided.

The two embodiments have in common that the overall systems require asmall chip area, and a short transmission path.

The second embodiment allows for simplified insertion of a preferablyexternally-accessible cavity, or of interlayers. An extremely compactanalysis system is thereby created which may be employed to analyzeliquids and gases present in the cavity.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross section through an optical transmitting and detectingdevice; and

FIG. 2 is a cross section through an optical transmitting and detectingdevice that includes a cavity in the light transmission path.

DETAILED DESCRIPTION OF THE INVENTION

An optical transmitting and detecting device 1 shown in FIG. 1 has, on asubstrate 2, a sequence of layers for a transmitting element 3, andlocated above said element a sequence of layers for a detecting element4.

To create transmitting element 3, an epitaxial layer 5 of a seconddoping type is applied to the heavily-doped silicon substrate 2 of afirst doping type. The blocking layer between these two layers is theregion of light emission.

After an intrinsic layer 6 has been applied to epitaxial layer 5, alayer 10 of the same doping type is diffused in, and extends up toepitaxial layer 5 of transmitting element 3. This the first pole. In theembodiment, the anode of transmitting element 3 is routed to the topside of the chip.

In this embodiment, the metallized bottom side of silicon substrate 2forms the cathode terminal 20 of transmitting element 3. This terminalcontact may also be routed to the top side if required.

In the embodiment, detecting element 4 is formed by a pin diodeincluding intrinsic layer 6 and diffusion zones 7, 8 spaced laterallyrelative to each other therein. To produce detecting element 4,intrinsic layer 6 is applied to epitaxial layer 5 of the transmittingelement, and the two zones 7 and 8 are diffused in. To obtain theelectrical insulation of transmitting element 3 from detecting element4, a dielectric interlayer 9, 9 a is inserted between these elements.The interlayer is formed by an oxide layer.

The horizontal interlayer section 9 is formed by oxygen-implantationaccording to the SIMOX process.

The lateral interlayer sections 9 a are formed by trench etching andtrench sealing.

The top side of intrinsic layer 6 is provided with an oxide layer 11 asa protective coating. Finally, the surface is provided through aconventional bonding process with terminal contacts, the metallized topside of layer 10 forming the anode terminal 12 of transmitting element3. The metallized top sides of the two diffusion zones 7 and 8 form thecathode terminal 13 and the anode terminal 14 for detecting element 4. Aclearly seen feature is that anode terminal 14 extends over the rearside of detecting element 4, thereby forming a metal layer acting as areflector 15. This metal layer enhances the efficiency of the detectingelement.

Reflector 15 may also be isolated from the other metallic contactcoatings.

If multiple transmitting and detecting devices 1 are arrangedsequentially on a common substrate, an oxide layer 16 is provided forsystem isolation, the layer being formed by trench etching or trenchsealing and extending from the top side of the chip beyond blockinglayer 17 of transmitting element 3.

Oxide layer 16 is added simultaneously with interlayer 9 a. During thetrench-etching process, etching is halted when the etching agent(etching gas) reaches the horizontal silicon oxide layer 9. The adjacentexternal trenches, which are produced at the same time, do not meet thistype of silicon oxide layer so that they can be made deeper and extendbeyond blocking layer 17, thus creating the system isolation.

The arrangement of transmitting element 3 and detecting element 4 withits sandwiched stacked layering creates a short light transmission pathin which practically only insulating interlayer 9 is located. The shorttransmission path means that only a low intensity is required for thelight emitted by transmitting element 3 (i.e., transmitting element 3may be operated with very low currents).

Detecting element 4 that is formed by a pin diode has an extremely highlight sensitivity, and thus a high efficiency. This also contributes tothe fact that the transmission side may be operated at a very low lightoutput. This aspect is aided by the metallic layer acting as reflector15 on the rear side of detecting element 4. As with contacts 12 through14, this layer may be composed of aluminum, or also possibly of gold oranother metal with high light reflectivity.

The transmitting and detecting device 1 may be provided sequentially ina multiple-element design in order to create a multichannel arrangementfor the light transmission.

It must also be mentioned that the detecting element 4 may also be inthe form of a phototransistor, photothyristor, photoresistor or similarlight-sensitive element. It is also possible on the detecting side tointegrate a following series-connected power switch so as to create, forexample, a photo MOS relay.

The transmitting and detecting device 1 in FIG. 1 may be employed as anoptocoupler for example.

FIG. 2 is a modified embodiment of a transmitting and detecting device 1a according to the invention. This embodiment first provides isolated,separate silicon chips for the transmitting element 3 a and detectingelement 4 a. After their production, these are joined in bonding region18 by chip bonding, with the light-emitting and light-detecting sidesfacing each other, thereby producing an embodiment approximatelycomparable to the embodiment of FIG. 1, wherein specifically here aswell the layers of transmitting element 3 and the layers of detectingelement 4 are arranged in a sandwiched stacked configuration.

In the embodiment of FIG. 2, a cavity 19 is located in the lighttransmission path between transmitting element 3 a and detecting element4 a, the cavity being preferably accessible from the outside. In theembodiments cavity 19 is formed by removing silicon from the epitaxiallayer 5. Since this side of the transmitting chip is freely accessiblebefore bonding to the detecting chip, cavity 19 may be produced simply.The insulating interlayer 9 formed by the oxide layer may also beproduced more easily before bonding due to the accessibility of thedetecting chip side than in the embodiment of FIG. 1. If necessary,insulating interlayer 9 may be dispensed with if a gas is present incavity 19 which creates a sufficient isolation and electrical insulationbetween the systems.

The coatings in the light transmission path may be provided for reasonsother than insulation purposes. One possibility is to create awavelength-selective filter layer, such as one composed of siliconnitride2, which transmits light only at a wavelength above 400 nm.

In addition to these wavelength-selective filter layers, almost anynumber of coatings may be applied to affect light transmission betweentransmitting element 3 a and detecting element 4 a.

The externally accessible cavity 19 may be filled or penetrated by agaseous or liquid media. It is thus possible to determine the type ofmaterial, employ the material as a filter, etc. Analyses of these mediamay thus be performed in which the measurement reaction occurring isdetected based on the specific material. For example, spectrometricanalyses may be performed or certain media monitored for turbidity. Thisfeature would enable monitoring of water quality, for example, or use asa fire detector in which smoke or the changed composition of the ambientair affects the transmission of light.

The arrangement of transmitting element 3 a, cavity 19 and detectingelement 4 a thus create a very compact analysis system in a preferablyconstructed chip.

It should be mentioned that a cavity in the light transmission path mayalso be provided in the embodiment of FIG. 1, one created for example byunderetching. Due to higher production costs for the embodiment of FIG.1, preferably one or more microcavities are provided, whereas in theembodiment of FIG. 2 with its accessibility to the inner sides of thetwo chips, one or more cavities of any size and shape may be provided.

It should also be mentioned here that the cavity 19 may be included notonly in the transmitting chip but also in the detecting chip.

Since transmitting and detecting device 1 and 1 a are produced based onsilicon technology, they may be readily integrated into othersilicon-based systems. Since the systems are largely protected fromambient light, flip-chip assemble is also possible.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. Method for producing an optical transmitting and detecting deviceincluding a light-emitting transmitting element as well as a detectingelement to convert this light into an electrical quantity, thetransmitting and detecting element being incorporated into a siliconsubstrate, where the optical transmitting and detecting device ismonolithically incorporated into a common substrate having a stackedsequence of layers for the light-transmitting element and thelight-detecting element, and that an electrically insulating interlayeris inserted between the transmitting element and the detecting element.2. The method of claim 1, wherein an epitaxial layer of as second dopingtype is applied to a heavily doped silicon substrate of a first dopingtype, thus creating the light-transmitting element.
 3. The method ofclaim 2, wherein a sequence of layers is applied to the epitaxial layerof the light-transmitting element to form a detecting element, and thata dielectric interlayer is inserted between the layers of thelight-transmitting element and the layers of the detecting element. 4.The method of claim 3, wherein an intrinsic layer is epitaxially appliedto the epitaxial layer of the light-transmitting element to form thelight-detecting element, into which intrinsic layer laterally spacedzones of different doping types are diffused.
 5. The method of claim 4,wherein layers of the second doping type are each diffused laterallyinto the layer of the detecting element, the layers extending up to theepitaxial layer of the same doping type of the transmitting element. 6.The method of claim 5, wherein a first oxide layer forming thedielectric interlayer is created by oxygen implantation, and thatsubsequent a second oxide layer extending up to the surface andsurrounding the detecting element is created by trench etching or trenchsealing.
 7. The method of claim 6, wherein a plurality of transmittingand detecting systems are produced adjacent to one another on a wafer.8. The method of claim 7, wherein an oxide layer extending from the topside of the chip and beyond the blocking layer of the transmittingelement is created to separate adjacent transmitting and detectingsystems, by trench etching and trench sealing, with the incorporation ofthe oxide layer surrounding the detecting element.
 9. The method ofclaim 8, wherein at least one, externally-accessible cavity isincorporated between the sequence of layers of the light-transmittingelement and the sequence of layers of the light-detecting element.
 10. Amethod for producing an optical transmitting and detecting deviceincluding a light-emitting transmitting element as well as a detectingelement to convert this light into an electrical quantity, thetransmitting and detecting element being incorporated into a siliconsubstrate, where the light-transmitting element and the light-detectingelement are each incorporated into a silicon substrate, that at leastone of the two thus-formed transmitting and detecting elements ismodified at the light-emitting or light-detecting side, that throughthis modification a cavity is inserted in at least one of these sides,and that the elements are joined with the light-emitting andlight-detecting sides facing each other.
 11. The method of claim 10,wherein the transmitting element and the detecting element are joined bybonding to each other in a sandwiched stacked configuration.
 12. Themethod of claim 10, wherein to create the transmitting element, anepitaxial layer of a second doping type is applied to a heavily dopedsilicon substrate.
 13. The method of claim 10, wherein a multiplicity oftransmitting elements are created on a first wafer, and a multiplicityof detecting elements are created on a second wafer, and that the twowafers are joined with their respective elements by wafer bonding. 14.The method of claim 13, wherein the externally-accessible cavity isinserted in the transmitting element on the side within the lighttransmission region facing the detecting element.
 15. The method ofclaim 10, wherein the cavity is filled with a gaseous or liquid medium.16. The method of claim 15, where at least one insulating layer and/orat least one wavelength-selective filter layer, is inserted into thesides of the transmitting elements and detecting elements facing eachother.
 17. The method of claim 16, wherein the terminal contacts for thelight-detecting element and the light-transmitting element are producedby diffusion technology and metallization.
 18. The method of claim 17,wherein during metallization of the terminal contacts, a metal coatingacting as a reflector and covering the rear side of the detectingelement is applied.
 19. The method of claim 10, wherein thelight-detecting element is coupled to an integrated power switch.
 20. Anoptical transmitting and detecting device including a light-emittingtransmitting element as well as a detecting element to convert lightinto an electrical quantity, the transmitting and the detecting elementbeing incorporated into a silicon substrate, where the opticaltransmitting and detecting elements are arranged in a stackedconfiguration in a common silicon substrate, and that a dielectricinterlayer is located between the transmitting element and the detectingelement.
 21. The optical transmitting and detecting device of claim 20,wherein one silicon chip each is provided for the light-transmittingelement and for the light-detecting element, that these chips are joinedwith the light-transmitting element and the light-detecting sides facingeach other.
 22. The optical transmitting and detecting device of claim20, wherein the light-detecting element comprises a pin diode.
 23. Theoptical transmitting and detecting device of claim 20, wherein saiddevice is configured and arranged as an optocoupler, an includes a powerswitch on the detector side.
 24. The optical transmitting and detectingdevice of claim 20, wherein said device has at least oneexternally-accessible cavity between the transmitting elements and thedetecting element, and is configured and arranged as an analysis systemfor spectrometric analyses.
 25. The optical transmitting and detectingdevice of claim 20, wherein said device has at least one externallyaccessible cavity between the light-transmitting elements and thelight-detecting element, and is configured and arranged as a firedetector.